It is frequently necessary to incorporate admixtures (e.g., fillers, reinforcing agents, accelerators and antioxidants) into a viscoelastic material (e.g., rubber or polymer compounds) in order to obtain the desired properties for a particular application. In this regard, various admixtures are mixed into the viscoelastic material in mixers, masticating machines, or on roll mills. These substances may include carbon black (for improved abrasion and wear resistance, tensile strength, tear resistance, modulus and hardness), silica, calcium carbonate, clay, oils (for better workability of the mixture), paraffin (for better resistance to light), antioxidants (e.g., aromatic amines or phenol derivatives), activators (e.g., zinc oxide), and various organic and inorganic coloring substances. Moreover, in order to speed up the vulcanization process and to improve the properties of vulcanizates, various accelerators may be added (e.g., dithiocarbamic acid derivatives, mecapto benzothiazole derivatives, diphenylguanidine, etc.). It should be understood that the term "admixture" as used herein includes additives and compounding ingredients.
When a viscoelastic material is processed (e.g., a mixing or extrusion process) work is put into the material, causing the temperature of the material to rise due to the poor thermoconductivity of the viscoelastic material. This heating process is referred to as "viscous heating." Viscous heating of a viscoelastic material during a mixing or extrusion process can have a significant effect on the properties of the viscoelastic material. In this regard, the elevated temperature of the material causes a decrease in the viscosity of the material. As a result, the material is unable to properly disperse fillers or other admixtures.
Different admixtures (e.g., fillers, reinforcing agents, accelerators and antioxidants) will cause different degrees of viscous heating depending on the type of admixture, its average particle size and distribution, primary and secondary structure, and particle shape. If the viscous heating is too great, a batch of viscoelastic material may rise in temperature too rapidly forcing the batch to be discharged before adequate dispersion is achieved. Consequently, an additional pass in an internal mixer may be required, which increases the total mixing time and cost.
For example, the use of carbon black will impart higher hysteresis and heat buildup to the vulcanizate as well as significantly increasing viscous heating during the mixing and processing of a viscoelastic compound. Accordingly, it would be very useful to be able to measure accurately and study the different degrees of viscous heating imparted by different carbon blacks to a rubber batch. Usually in a rubber mixing operation, a particular compound may require two, three, or more "passes" in a factory internal mixer before an acceptable state of dispersion is achieved. A major limiting factor for the time length of a mixing cycle is determined from viscous heating. If a reinforcing filler such as certain carbon blacks quickly raise the temperature of the batch to very high levels, the viscosity of the batch drops so low at this high temperature that little useful mixing action will take place. Therefore, there becomes a need for multiple passes. It would be useful to be able to compare the viscous heating effects of different admixtures in order to predict the useful mixing time possible per batch.
In the prior art, there are many well known instruments for determining various properties of viscoelastic materials (e.g., rubber and like materials). These instruments include such apparatus commonly referred to as Moving Die Rheometers (MDR), Rubber Process Analyzers (RPA), Oscillating Disk Rheometers (ODR), and Mooney Viscometers. These instruments apply a rotational shear to a sample of viscoelastic material and measure the resulting torque. It should be understood that the applied rotational shear may be oscillatory or continuous. In the case of an MDR or RPA, a sample of viscoelastic material to be tested is enclosed in a cavity formed between two opposing dies, and the rotational shear is applied to the sample by rotating one die, while the other die remains stationary, and the torque required to apply the shear is measured. In the case of an ODR or Mooney Viscometer a sample of viscoelastic material to be tested is enclosed in a cavity formed between two opposing dies, rotational shear is applied to the sample by means of a rotor embedded in the sample, and the torque required to apply the shear is measured. In U.S. Pat. Nos. 3,479,858; 4,343,190; and 4,552,025, the force is applied by rotation of one die relative to the other, and the measurements made are of the torque required to apply the shearing force or of the torque induced in the first die (reaction torque) when the second (driven) die is rotated.
While such existing instruments have been used to measure such items as elastic torque, viscous torque, and complex torque, none of these instruments can measure viscous heating caused by application of a shearing force to the sample material. The present invention provides a method and apparatus for measuring the viscous heating of a viscoelastic material.